The aim of this study was to investigate the polymicrobial communities in an adult Cystic Fibrosis population stratified by gender and the most common CFTR mutation, F508del. In this pilot study, DNA was extracted from sputum samples of 29 adult patients (16 male: 13 female) with an F508del mutation in a stable clinical state. Universal primers were used to amplify DNA from bacterial and fungal communities and the resulting fragments were analysed by denaturing gradient gel electrophoresis. Bacterial profiles showed a significant effect of gender (P = 0.046) and P. aeruginosa carriage (P = 0.034) on community structure. Bacterial communities were found to be randomly assembled. Fungal community analysis found that F508del homozygous patients had a greater diversity than heterozygous patients (P = 0.007). This study indicates that the bacterial lung communities of adult CF patients are randomly assembled but have distinct gender based differences. Furthermore, the fungal communities colonising the CF lung are more diverse in F508 homozygotes. This is the first paper to identify a reduced bacterial diversity in female patients with CF and to implicate more severe CFTR genotypes with increased risk of infection with multiple fungal species.